1. Field of the Invention
The present invention relates to an integrated circuit (IC) having a trimless resistor.
2. Description of the Prior Art
Integrated circuits employ resistors formed by a variety of materials and processes. For example, a deposited doped polysilicon resistor that overlies a semiconductor substrate may be used. In other cases, a doped region in the semiconductor substrate itself may be used as the resistor. A heavily doped region formed by the same diffusion process that forms the source and drain regions may be used when a relatively low resistivity is desired. A less heavily doped tub region may be used as the resistor where a higher resistivity is desired. For example, an n-doped tub region may be used to form an "NTUB" resistor when a relatively large resistance value is desired. However, the NTUB resistor has a relatively poor temperature coefficient. For example, in one typical integrated circuit fabrication process implemented in 0.5 micron CMOS technology, the resistance value increases by 55 percent over a temperature range of from 10 degrees to 110 degrees centigrade.
It is known in the integrated circuit art to trim resistors to obtain a relatively more precise value. For example, "trim links" are resistive regions in series that can be selectively bypassed during a trimming operation to obtain the desired resistance value. The trimming may be accomplished by application of a large electrical current in some cases. However, that requires extra bondpads on the IC for supplying the current, which is undesirable in many cases. However, in the case of the above-noted resistor types, and especially the tub resistor types, the temperature coefficient is so poor that trimming these resistors using trim links does not achieve the desired result, since trimming is done at only one temperature. In many cases, relatively constant resistor values as a function of temperature are desired, as in the case of resistors used in setting filter coefficients in analog circuits, for example.